When hemihydrate gypsum is mixed with water to prepare hemihydrate gypsum slurry, it is hydrated to become hardened easily as dihydrate gypsum. Therefore, it is widely used as a raw material for gypsum boards and medical casts.
When the hemihydrate gypsum is mixed with water, a hydration reaction begins accompanied by heat generation after the passage of a significant induction period so that the hemihydrate gypsum gradually becomes hardened. The length of the induction period and the length of time required until the end of the hydration reaction differ according to the particle diameter of the hemihydrate gypsum as a raw material, the mixing ratio of it and water, and the existence of an additive. For example, when the hemihydrate gypsum as a reagent and water are mixed together in a ratio (weight ratio) of 1:2 to carry out a hydration reaction in a laboratory, the induction period is about more than a dozen minutes and the hydration reaction ends in about 1 to 2 hours.
Since the hardening time becomes shorter as the hydration reaction rate is higher, thereby improving production efficiency, the hardening of the hemihydrate gypsum is generally carried out by adding a hydration reaction accelerator. As the above hydration reaction accelerator is used an alkali metal salt of sulfuric acid.
Both the induction period and the hardening time of the hemihydrate gypsum slurry comprising a hydration reaction acceleration are shortened. That is, when the hydration reaction accelerator is added to shorten the hardening time, the induction period is shortened together with the hardening time, whereby it is impossible in principle to achieve an induction time long enough for the work of casting into a form and a hardening time short enough from the viewpoint of work efficiency at the same time. Further, even when a very small amount of a known hydration reaction accelerator is added, its effect is not obtained. Only when a certain amount of the hydration reaction accelerator is added, its effect is abruptly developed. Therefore, it is actually extremely difficult to control the induction period to about 5 to 10 minutes by changing the amount of the hydration reaction accelerator.
To harden the hemihydrate gypsum, a certain amount of water must be added so as to secure the flowability of initial slurry. At this point, it is known that the content of water in the slurry and the time until the hardened product is completely dried correlate with each other. When the amount of water to be removed from the hardened product by evaporation differs by 5 wt %, the production cost greatly differs. Therefore, it is desired to reduce the content of water in the slurry as much as possible. However, when the content of water in the slurry is reduced, the flowability of the slurry is impaired. Therefore, there is limitation to the reduction of the content of water in the slurry.
Thus, there has been unknown hemihydrate gypsum having an induction time of several minutes which is preferred for the work of casting into a form and a short hardening time of several tens of minutes which is preferred for the improvement of work efficiency and exhibiting high flowability even when the content of water in the slurry is reduced.